Co2 recovery unit and co2 recovery method

ABSTRACT

A CO2 recovery unit includes a CO2 absorber that brings a gas having a low CO2 concentration into countercurrent contact with a CO2 absorbent to remove CO2 from the gas. The CO2 recovery unit further includes a first absorbent circulation line that supplies a CO2 absorbent from a first CO2 absorption section as a first circulation solution to an upper side of a first CO2 absorption section; a second absorbent circulation line that supplies a CO2 absorbent from a second CO2 absorption section as a second circulation solution to an upper side of a second CO2 absorption section; and an absorbent discharge line that discharges a part of the first circulation solution from the first absorbent circulation line and supply the part of the first circulation solution as a discharged solution to the second absorbent circulation section.

FIELD

The present invention relates to a CO₂ recovery unit and a CO₂ recoverymethod.

BACKGROUND

Various methods have been proposed so far to recover and remove acidgases, especially CO₂, contained in flue gases from boilers, forexample. One of such methods is to bring CO₂ into contact with a CO₂absorbent made of an amine aqueous solution to remove and recover CO₂contained in the flue gases.

For example, the method involves use of the CO₂ absorbent in a CO₂absorber to absorb CO₂ contained in the flue gases for removal of CO₂from the flue gases, followed by regeneration of the CO₂ absorbent byreleasing CO₂ absorbed in the CO₂ absorbent and regenerate the CO₂absorbent in a regenerator, and circulation of the regenerated CO₂absorbent into the CO₂ absorber for reuse of the CO₂ absorbent to removeCO₂ from the flue gases again. The CO₂-absorbing CO₂ absorbent is heatedin the regenerator with use of steam flowed from a reboiler to releaseCO₂ for recovery of CO₂ at a high purity. In this method, it is proposedthat a part of the CO₂ absorbent is discharged at a lower part andreturned to at least one portion(s) between an upper CO₂ absorbentsupply part and a lower gas supply part in the CO₂ absorber to increasean amount of CO₂ absorbed from the flue gas while reducing a reboilerheat duty required for the regeneration of the CO₂ absorbent (forexample, Patent Literature 1). It is proposed that the CO₂ absorber isprovided at its absorption section with a plurality of stages to allow apart of a discharged solution to be returned to an upper side above aposition for discharging a circulated solution at the lower stage in theabsorption section to increase the amount of CO₂ absorbed from the fluegas while reducing an energy required for the regeneration of the CO₂absorbent, as well (for example, Patent Literature 2).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Translation of PCT Application PublicationNo. 2013-512088

Patent Literature 2: International Publication No. 09/104744

SUMMARY Technical Problem

However, the current CO₂ recovery unit has a problem of requiring alarge amount of heat in reboiling for reuse of a CO₂ absorbent becauseof difficulties in CO₂ release at a CO₂ regenerator and reduction ofsteam energy required for regeneration of the CO₂ absorbent.

Furthermore, although the CO₂ content in a flue gas is 10 to 15% byvolume (represented simply by “%”, hereinafter), there is a demand for aCO₂ recovery unit and a CO₂ recovery method that are capable of reducingthe amount of heat in reboiling for the reuse of the CO₂ absorbent evenwhen CO₂ is recovered from gases under nearly atmospheric pressurecontaining a subtle amount of CO₂ other than the flue gas as well as avariety of gases with low CO₂ contents in gases having CO₂concentrations of 10% or less flowed from gas turbine gas and the like,for example.

In light of the aforementioned circumstances, the present invention hasan object to provide a CO₂ recovery unit and a CO₂ recovery method thatare capable of reducing an amount of heat in reboiling for reuse of aCO₂ absorbent to recover CO₂ from a variety of gases with low CO₂contents.

Solution to Problem

A CO₂ recovery unit according to a first aspect of the present inventionincludes: a CO₂ absorber that brings a gas having a low CO₂concentration into countercurrent contact with a CO₂ absorbent to removeCO₂; an absorbent regenerator that performs a heat exchange between arich solution containing absorbed CO₂ and water vapor flowed from areboiler to regenerate the CO₂ absorbent; a rich solution supply linethat discharges a rich solution containing absorbed CO₂ from a bottompart of the CO₂ absorber and supplies the rich solution to an upper sideof the absorbent regenerator; and a lean solution supply line thatdischarges a lean solution obtained by releasing CO₂, from a bottom partof the absorbent regenerator and supplies the lean solution to an upperside of the CO₂ absorber. The CO₂ absorber includes a CO₂ absorptionsection with at least two or more stages. The CO₂ recovery unit furtherincludes: absorbent circulation lines that each discharge the CO₂absorbent from a lower side of one of the stages in the CO₂ absorptionsection and supply the CO₂ absorbent as a circulation solution to anupper side of the corresponding stage used for the discharge in the CO₂absorption section; and an absorbent discharge line that discharges apart of the circulation solution from the absorbent circulation line andsupply the part of the circulation solution as a discharged solution toa stage immediately below the stage used for the discharge.

A CO₂ recovery method according to a second aspect of the presentinvention includes: a CO₂ absorption step of bringing a gas having a lowCO₂ concentration into countercurrent contact with a CO₂ absorbent toremove CO₂; an absorbent regeneration step of performing a heat exchangebetween water vapor and a rich solution containing the absorbed CO₂ toregenerate the CO₂ absorbent; and a step of discharging a rich solutioncontaining CO₂ that is absorbed at the CO₂ absorption step, supplyingthe rich solution to the absorbent regeneration step, discharging a leansolution that is obtained by releasing CO₂ from a tower bottom part atthe absorbent regeneration step, and supplying the lean solution to theCO₂ absorption step to recover CO₂ in the gas. The CO₂ absorption stepincludes an absorbent circulation step of, in a CO₂ absorption sectionwith at least two or more stages, discharging the CO₂ absorbent fromlower sides of the stages in the CO₂ absorption section and supplyingthe CO₂ absorbent as a circulation solution to upper sides of thecorresponding stages used for the discharge in the CO₂ absorptionsection; and an absorbent discharge step of discharging a part of thecirculation solution from the absorbent circulation step and supplyingthe part of the circulation solution as a discharged solution to a stageimmediately below the stage used for the discharge.

Advantageous Effects of Invention

According to the present invention, an amount of heat can be reduced inreboiling for CO₂ recovery to recover CO₂ from a variety of gases withlow CO₂ contents.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view representing a configuration of a CO₂recovery unit according to a first embodiment.

FIG. 2 is a schematic view representing a configuration of a CO₂recovery unit according to a second embodiment.

FIG. 3 is a schematic view representing a configuration of a CO₂recovery unit according to a third embodiment.

FIG. 4 is a graph representing CO₂ concentration ratios in a richsolution in test example 1.

FIG. 5 is a graph representing CO₂ recovery amount ratios in testexample 1.

FIG. 6 is a graph representing reduction percentages (%) of reboilerheat duty in an absorbent regenerator in test example 1.

FIG. 7 is a graph representing CO₂ concentration ratios in a richsolution in test example 2.

FIG. 8 is a graph representing CO₂ recovery amount ratios in testexample 2.

FIG. 9 is a graph representing reduction percentages (%) of reboilerheat duty in an absorbent regenerator in test example 2.

FIG. 10 is a graph representing CO₂ concentration ratios in a richsolution in test example 3.

FIG. 11 is a graph representing CO₂ recovery amount ratios in testexample 3.

FIG. 12 is a graph representing reduction percentages (%) of reboilerheat duty in an absorbent regenerator in test example 3.

FIG. 13 is a graph representing CO₂ concentration ratios in a richsolution in test example 4.

FIG. 14 is a graph representing CO₂ recovery amount ratios in testexample 4.

FIG. 15 is a graph representing reduction percentages (%) of reboilerheat duty in an absorbent regenerator in test example 4.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention aredescribed in detail with reference to the accompanying drawings. Thepresent invention is not limited to these embodiments, andmodifications, additions, and omissions can be made by a person skilledin the art without departing from the spirit and scope stated in theclaims.

First Embodiment

FIG. 1 is a schematic view representing a configuration of a CO₂recovery unit according to a first embodiment. As illustrated in FIG. 1, a CO₂ recovery unit 10A is a unit for recovering CO₂ in an introducedgas (e.g., air) 11A under nearly atmospheric pressure with a lowconcentration of CO₂, for example. The CO₂ recovery unit 10A is providedwith: a CO₂ absorber 14 configured to bring the introduced gas 11A intocountercurrent contact with a lean solution 13A of a CO₂ absorbent forallowing CO₂ contained in the introduced gas 11A to be absorbed in theCO₂ absorbent and then removed; and an absorbent regenerator 15 that isformed at a subsequent stage of the CO₂ absorber 14 to release CO₂ froma rich solution 13C of CO₂ absorbent containing absorbed CO₂ and thenregenerate the lean solution 13A.

In this CO₂ recovery unit 10A, the CO₂ absorbent is circulated betweenthe CO₂ absorber 14 and the absorbent regenerator 15. The lean solution13A of the CO₂ absorbent obtained as a result of releasing CO₂ absorbsCO₂ from the gas in the CO₂ absorber 14 to become the rich solution 13C.This rich solution 13C is supplied to the absorbent regenerator 15. Thesupplied rich solution 13C release CO₂ in the absorbent regenerator 15,and is regenerated as the lean solution 13A to be subsequently suppliedto the CO₂ absorber 14. Here, the CO₂ absorbent is a generic termreferring to: the lean solution 13A obtained by releasing CO₂; asemi-rich solution 13B that absorbs a part of CO₂ in the gas; and therich solution 13C that contains CO₂ absorbed from the gas and isdischarged from the CO₂ absorber 14. Each of the names is selecteddepending on circulating positions in the CO₂ recovery unit 10A and theCO₂ content ratio.

The CO₂ absorbent usable in the present invention is not particularlylimited. Examples of the CO₂ absorbent include amine compounds such asalkanolamines and hindered amines with alcoholic hydroxyl groups.Examples of such alkanolamines include monoethanolamine, diethanolamine,triethanolamine, methyl diethanolamine, diisopropanolamine, anddiglycolamine. Among them, monoethanolamine (MEA) is generallypreferred. Examples of such hindered amines with alcoholic hydroxylgroups include 2-amino-2-methyl-1-propanol (AMP), 2-(ethylamino)-ethanol(EAE), 2-(methylamino)-ethanol (MAE), and 2-(diethylamino)-ethanol(DEAE).

A rich solution supply line L₁ is provided to supply the rich solution13C containing CO₂ absorbed in the CO₂ absorber 14 to an upper side ofthe absorbent regenerator 15 between a bottom 14 b of the CO₂ absorber14 and a top 15 a of the absorbent regenerator 15. The rich solutionsupply line L₁ is provided with: a rich solution pump 51 configured tosupply the rich solution 13C containing CO₂ absorbed in the CO₂ absorber14 to the absorbent regenerator 15; and a rich/lean solution heatexchanger 52 configured to heat the rich solution 13C using the leansolution 13A that is obtained as a result of releasing CO₂ by heating inthe absorbent regenerator 15.

A CO₂ absorption section 141 includes: a first CO₂ absorption stage 141A(referred to as “first absorption stage”, hereinafter) arranged toabsorb CO₂ from the introduced gas 11A; and a second CO₂ absorptionstage 141B (referred to as “second absorption stage”, hereinafter)located below the first absorption stage 141A that are disposed insidethe CO₂ absorber 14 in its height direction.

A solution reservoir 143A and a chimney tray 143B are provided betweenthe first absorption stage 141A and the second absorption stage 141B.The solution reservoir 143A is arranged to store the semi-rich solution13B that is flowed down from upper portions of the first absorptionstage 141A to reach lower portions of the first absorption stage 141A.

The solution reservoir 143A is provided with a first absorbentcirculation line L₁₁ that is arranged to discharge a whole amount of thesemi-rich solution 13B stored in the solution reservoir 143A through adischarge position A of the CO₂ absorber 14 and introduce the dischargedsolution into an introduction position B located at the upper portion ofthe first absorption stage 141A that is the same stage as that for thedischarge.

The first absorbent circulation line L₁₁ is provided with a semi-richsolution pump 25 to circulate the semi-rich solution 13B into the upperportion of the first absorption stage 141A. The first absorbentcirculation line L₁₁ is provided with a circulation flow control valveV₁ that controls a circulation flow rate of the circulated semi-richsolution 13B.

A front end of a lean solution supply line L₂ is connected to the firstabsorbent circulation line L₁₁ at a connection position H, such that thelean solution 13A regenerated in the absorbent regenerator 15 is mixedwith the circulated semi-rich solution 13B (circulation solution 13B) toform a mixture (13A/13B). The mixture is introduced into an upper stageliquid distributor 140A. The front end of the lean solution supply lineL₂ may be connected directly to the upper stage liquid distributor 140Aat the upper portion of the first absorption stage 141A withoutconnected to the first absorbent circulation line L₁₁ such that the leansolution 13A and the circulated semi-rich solution 13B (circulationsolution 13B) are individually introduced into the upper stage liquiddistributor 140A.

A second absorbent circulation line L₁₂ is provided to be connected at aliquid reservoir 144 located at a bottom of the CO₂ absorber todischarge a whole amount of the rich solution 13C stored in the liquidreservoir 144 through a discharge position C of the CO₂ absorber 14 andintroduce the discharged solution into an introduction position Dpositioned at an upper part of the second absorption stage 141B that isthe same stage as the stage for the discharge.

A part of the rich solution 13C is separated as a circulation solution13C-1 at a discharge position E by the second absorbent circulation lineL₁₂. The second absorbent circulation line L₁₂ is connected at thedischarge position E to a base end of the rich solution supply line L₁.The rich solution 13C discharged is supplied through the rich solutionsupply line L₁ to the absorbent regenerator 15. The rich solution supplyline L₁ is provided with a rich solution flow control valve V₂ tocontrol the amount of the rich solution 13C supplied to the absorbentregenerator 15. The second absorbent circulation line L₁₂ is providedwith a circulation flow control valve V₃ to control the circulation flowrate of the circulated circulation solution 13C-1.

An absorbent discharge line L₂₁ is provided to discharge a part of thesemi-rich solution 13B (circulation solution 13B) circulated from thefirst absorbent circulation line L₁₁ as a discharged solution 13B-1 at adischarge position F and supply the discharged solution 13B-1 to thesecond absorption stage 141B that is the adjacent stage immediatelybelow the stage for the discharge.

In the present embodiment, the front end of the absorbent discharge lineL₂₁ is connected at a connection position G to the second absorbentcirculation line L₁₂, such that the discharged solution 13B-1 is mixedwith the circulation solution 13C-1 to form the mixture (13B-1/13C-1).The mixture is supplied through a single line to the introductionposition D located at the upper portion of the second absorption stage141B. The front end of the absorbent discharge line L₂₁ may be connecteddirectly to the lower stage liquid distributor 140B at the upper portionof the second absorption stage 141B without connected to the secondabsorbent circulation line L₁₂ such that the discharged solution 13B-1and the circulation solution 13C-1 are individually introduced into thelower stage liquid distributor 140B.

In addition, a discharge flow control valve V₄ is provided in theabsorbent discharge line L₂₁ to control the discharge flow rate of thedischarged solution 13B-1.

In the first absorption stage 141A, the lean solution 13A regenerated inthe absorbent regenerator 15 and the circulated semi-rich solution 13B(circulation liquid 13B) are supplied as the CO₂ absorbents. Thesupplied lean solution 13A and the circulated semi-rich solution 13B(circulation liquid 13B) are distributed from the upper stage liquiddistributor 140A that is located at the upper portion of the firstabsorption stage 141A, and then flowed down within a packed bed. Here,the semi-rich solution 13B as the circulation solution has a higher CO₂concentration than the lean solution 13A, but is still capable ofabsorbing CO₂. Thus, the semi-rich solution 13B can be reused in thefirst absorption stage 141A to absorb CO₂ from the introduced gas 11B.

To the second absorption stage 141B, the circulation solution 13C-1 issupplied together with the discharged solution 13B-1 of the semi-richsolution 13B containing CO₂ absorbed partially from the introduced gas11B in the first absorption stage 141A. The supplied discharged solution13B-1 and the circulation solution 13C-1 are distributed from the lowerstage liquid distributor 140B at the upper portions of the secondabsorption stage 141B and flowed down within the packed bed. Here, therich solution 13C as the circulation solution has a higher CO₂concentration than the semi-rich solution 13B, but is still capable ofabsorbing CO₂. Thus, the rich solution 13C can be reused in the secondabsorption stage 141B to absorb CO₂ from the introduced gas 11A.

In the first absorption stage 141A of the CO₂ absorber 14, theintroduced gas 11B containing CO₂ is brought into countercurrent contactwith the mixture of the lean solution 13A made of the aminecompound-based CO₂ absorbent and the circulated semi-rich solution 13B(circulation solution 13B) that are introduced into the tower. As aresult, the gas introduced into the tower becomes an outlet gas 11C byremoving CO₂ in the gas while the lean solution 13A becomes thesemi-rich solution 13B.

In the second absorption stage 141B, the semi-rich solution 13B(discharged solution 13B-1 and the circulation solution 13C-1) partiallyabsorbing CO₂ is brought into countercurrent contact with the introducedgas 11A that contains CO₂ and is introduced from a bottom side of thetower, allowing CO₂ to be absorbed into the semi-rich solution 13B fromthe introduced gas 11A by a chemical reaction. As a result, theintroduced gas 11A becomes the introduced gas 11B with a reduced CO₂concentration by removing CO₂ in the gas while the semi-rich solution13B becomes the rich solution 13C with higher amount of absorbed CO₂.This arrangement allows the introduced gases 11A and 11B containing CO₂to be flowed through the first absorption stage 141A and the secondabsorption stage 141B, removing CO₂ to provide a decarbonated outlet gas11C.

A mist eliminator 145 is installed inside a tower top 14 a to capturemist contained in the decarbonated outlet gas 11C and then discharge thegas outwardly from the tower top 14 a of the CO₂ absorber 14.

The absorbent regenerator 15 is provided at its interior with a packedbed 151 to release CO₂ with aid of water vapor from the rich solution13C containing absorbed CO₂. The absorbent regenerator 15 is provided atthe vicinity of a tower bottom 15 b with a circulation line L₄ thatcirculates therethrough a part of the lean solution 13A flowed down tothe tower bottom 15 b. The circulation line L₄ is provided with areboiler 31 that generates water vapor by indirect heating of the leansolution 13A with use of saturated steam S, and a control valve 32 thatcontrols the amount of saturated steam S supplied to the reboiler 31.The saturated steam S becomes steam condensate W₄ after the heating.

The absorbent regenerator 15 is provided at its top 15 a with a gasdischarge line Ls to discharge CO₂ gas 41 together with water vapor. Thegas discharge line Ls is provided with a condenser 42 that condenses thewater vapor discharged together with the CO₂ gas 41 into water, and aseparation drum 43 to separate a CO₂ gas 44 from condensed water W₅. TheCO₂ gas 44 separated from the condensed water W₅ is discharged outwardlyfrom an upper portion of the separation drum 43. A condensed water lineL₆ is provided between the bottom part of the separation drum 43 and thetop 15 a of the absorbent regenerator 15 to supply the condensed waterW₅ separated in the separation drum 43 to the top 15 a of the absorbentregenerator 15. The condensed water line L₆ is provided with a condensedwater circulation pump 45 to supply the condensed water W₅ separated inthe separation drum 43 to the top 15 a of the absorbent regenerator 15.

The lean solution supply line L₂ is provided between the bottom 15 b ofthe absorbent regenerator 15 and the top 14 a of the CO₂ absorber 14 tosupply the lean solution 13A of the CO₂ absorbent from the bottom 15 bof the absorbent regenerator 15 to the upper side of the CO₂ absorptionsection 141. The lean solution supply line L₂ is provided with: arich/lean solution heat exchanger 52 that heats the rich solution 13Ccontaining the absorbed CO₂ with use of the lean solution 13A that isobtained by heating with water vapor in the absorbent regenerator 15 forthe removal of CO₂; a lean solution pump 54 that supplies the leansolution 13A from the bottom 15 b of the absorbent regenerator 15 to theupper portion of the CO₂ absorption section 141; and a cooling part 55that cools the lean solution 13A of the CO₂ absorbent down to apredetermined temperature.

In the present embodiment, this cooling part 55 is configured to coolthe lean solution 13A down to the same temperature as or a temperatureclose to a gas temperature (room temperature: e.g., 25° C.) of theintroduced gas 11A introduced into the CO₂ absorber 14.

Next, the overall operation of the CO₂ recovery unit 10A according tothe present embodiment is described. For example, the introduced gas(atmospheric gas) 11A containing a subtle amount of CO₂ is introducedinto the interior of the CO₂ absorber 14 through the bottom 14 b of theCO₂ absorber 14 and flows upward inside the tower. The gas introducedinto the CO₂ absorber 14 is brought into countercurrent contact with theCO₂ absorbent containing the amine compound such as alkanolamine in thefirst absorption stage 141A and second absorption stage 141B of the CO₂absorption section 141, such that CO₂ is absorbed from the gas into theCO₂ absorbent and then removed for the purpose of providing thedecarbonated outlet gas 11C.

The decarbonated outlet gas 11C is discharged outwardly from the top 14a of the CO₂ absorber 14.

After the absorption of CO₂ in the CO₂ absorber 14, the rich solution13C of the CO₂ absorbent is flowed in the rich solution supply line L₁to pass through the rich/lean solution heat exchanger 52 for heatexchange with the lean solution 13A and then supplied to the top side ofthe absorbent regenerator 15 with the aid of the rich solution pump 51.

The rich solution 13C of the CO₂ absorbent supplied to the absorbentregenerator 15 is allowed to release CO₂ by the heat of the water vaporsupplied from the reboiler 31 while flowing downward inside the packedbed 151 of the absorbent regenerator 15, so as to become the leansolution 13A. In the absorbent regenerator 15, the solution iscirculated from the liquid reservoirs 151B of the chimney tray 151A intothe circulation line L₄ while heated by saturated steam S in thereboiler 31 to generate water vapor inside the absorbent regenerator 15.The lean solution 13A is obtained as a result of releasing CO₂ from therich solution 13C with the aid of the heat of this generated watervapor. After the heating, the saturated steam S becomes steam condensateW₄. The CO₂ gas 41 released together with the water vapor from the CO₂absorbent is cooled down for the condensation into water with use of thecondenser 42, separated from the condensed water W₅, and then dischargedas the CO₂ gas 44 outwardly through the upper portion of the separationdrum 43. The condensed water W₅ separated is supplied to the absorbentregenerator 15.

After discharged from the bottom 15 b of the absorbent regenerator 15,the lean solution 13A is flowed in the lean solution supply line L₂ topass through the rich/lean solution heat exchanger 52 for heat exchangewith the rich solution 13C and then supplied to the upper side of theCO₂ absorption section 141 of the CO₂ absorber 14 with the aid of thelean solution pump 54.

After supplied to the upper side of the CO₂ absorption section 141, thelean solution 13A absorbs CO₂ from the introduced gas 11B in the firstabsorption stage 141A to become the semi-rich solution 13B, which isdischarged at the discharge position A at the lower portion of the firstabsorption stage 141A and flowed into the first absorbent circulationline L₁₁. The discharged semi-rich solution 13B is supplied with the aidof the semi-rich solution pump 25 together with the lean solution 13A tothe introduction position B positioned at the upper side of the firstabsorption stage 141A.

In the second absorption stage 141B, CO₂ is absorbed from the introducedgas 11B to form the rich solution 13C. The rich solution 13C isdischarged at the discharge position C at the lower part of the secondabsorption stage 141B and flowed into the second absorbent circulationline L₁₂. The discharged rich solution 13C is separated at the dischargeposition E, as the circulation solution 13C-1. Then, the circulationsolution 13C-1 is supplied with the aid of the rich solution pump 51 tothe introduction position D at the upper portion of the secondabsorption stage 141B.

A part of the semi-rich solution 13B is separated at the dischargeposition F of the first absorbent circulation line L₁₁ by the absorbentdischarge line L₂₁. The discharged solution 13B-1 is mixed with thecirculation solution 13C-1 at the connection position G of the secondabsorbent circulation line L₁₂. This discharged solution 13B-1 and thecirculation solution 13C-1 are mixed with each other (13B-1/13C-1) andthen supplied through a single line to the introduction position D atthe upper portion of the second absorption stage 141B.

After separated from the circulation solution 13C-1 at the dischargeposition E, the rich solution 13C is supplied to the absorbentregenerator 15 through the rich solution supply line L₁.

Therefore, the configuration according to the present embodimentincludes: the first absorbent circulation line L₁₁ for supplying thesemi-rich solution 13B discharged at the lower side of the firstabsorption stage 141A in the CO₂ absorber 14 to the upper side of thesame stage 141A as the stage for the discharge; the second absorbentcirculation line L₁₂ for supplying the rich solution 13C discharged atthe lower side of the second absorption stage 141B of the CO₂ absorber14 to the upper side of the second absorption stage 141B that is thesame stage as the stage for the discharge; the absorbent discharge lineL₂₁ for separating the circulated semi-rich solution 13B (circulationsolution 13B) discharged at the lower side of the first absorption stage141A and then mixing the separated solution with the lean solution 13C-1that is discharged at the lower side of the second absorption stage 141Bthat is the adjacent stage immediately below the stage for thedischarge; the lean solution supply line L₂ for supplying the leansolution from the absorbent regenerator 15 to the upper portion of thefirst absorption stage 141A that is the highest stage of the CO₂absorber 14 and then mixing the supplied solution with the semi-richsolution 13B discharged at the lower side of the first absorption stage141A; and the rich solution supply line L₁ for flowing the rich solution13C from the bottom part of the CO₂ absorber 14 to the absorbentregenerator 15.

Thus, according to the first embodiment, after absorbing CO₂, thesemi-rich solution 13B and the rich solution 13C that contain absorbedCO₂ can be discharged and flowed outwardly through the lower sides ofthe first absorption stage 141A and the second absorption stage 14B,respectively, and supplied as the circulation solutions to the upperportions at the corresponding stages above the absorbent-dischargedportions. This allows the circulated circulation solutions to be reusedas the absorbents in the first absorption stage 141A and the secondabsorption stage 14B to recover CO₂ in the gas. As a result, withincrease of the CO₂ concentration in the CO₂ absorbent, it is possibleto improve the CO₂ recovery efficiency in the CO₂ absorber 14 andthereby increase the CO₂ recovery amount per unit volume of the CO₂absorbent. Furthermore, the CO₂ concentration in the rich solution 13Ccan be increased at the bottom 14 b of the CO₂ absorber 14, therebyallowing the rich solution 13C having an increased CO₂ concentration tobe supplied to the absorbent regenerator 15.

Here, when the two-stage configuration for the absorption section, thefirst absorption stage 141A and the second absorption stage 141B, as inthe present embodiment, the solution discharged from the circulationsolution 13C-1 is the rich solution 13C, while the rich solution supplyline L₁ functions as an absorbent discharge line to supply thedischarged rich solution 13C to the absorbent regenerator 15. In thecase of three-stage configuration for the absorption section, the firstabsorption stage 141A, the second absorption stage 141B, and a thirdabsorption stage, a solution discharged from the circulation solution13C-1, which is supplied from the second absorption stage 141B, issupplied as the discharged solution to the third absorption stageimmediately below the discharged stage of the second absorption stage.

As a result, the semi-rich solution 13B and the rich solution 13C arereused twice in the CO₂ absorber 14, enabling it to reduce the flow rateof the circulated CO₂ absorbent in the system circulating between theCO₂ absorber 14 and the absorbent regenerator 15. This can reduce theflow rate of the rich solution 13C supplied to the absorbent regenerator15, reducing the steam supply amount of the reboiler 31 used in theabsorbent regenerator 15 for the regeneration of the CO₂ absorbent, andthereby reducing heat energy consumption to improve energy efficiency.

In the present embodiment, even in the case of recovering CO₂ from a gascontaining a very subtle amount of CO₂ (e.g., air), the CO₂concentration can be increased in the rich solution 13C supplied to theabsorbent regenerator 15 so as to reduce the steam amount required inthe absorbent regenerator 15, thereby reducing the thermal energy aswell as achieving further energy saving in the process of regeneratingthe CO₂ absorbent.

Next, the separation ratios of the circulation solution to thedischarged solution in the CO₂ recovery unit 10A according to thepresent embodiment, is described.

In the present embodiment, regarding the flow rate (f1) of the semi-richsolution 13B (circulation solution 13B) circulated through the firstabsorbent circulation line L₁₁ from the lower portion of the firstabsorption stage 141A and the flow rate (f2) of the discharged solution13B-1 discharged via the absorbent discharge line L₂₁, the separationratio of the flow rates (referred to as “separation flow ratio”,hereinafter) is preferably 1:1 to 60:1, preferably 10:1 to 60:1, morepreferably 30:1 to 60:1.

Regarding the flow rate (f3) of the circulation solution 13C-1 flowedinto the second absorbent circulation line L₁₂ and the flow rate (f4) ofthe rich solution 13C flowed into the rich solution supply line L₁ fromthe bottom of the CO₂ absorber 14, the flow separation ratio of the flowrates is preferably 1:1 to 60:1, preferably 10:1 to 60:1, morepreferably 30:1 to 60:1.

Hereinafter, preferred test examples exhibiting effects of theseparation flow ratio according to the present embodiment are described.The present invention is not limited to these examples. FIGS. 4 to 9 inthese test examples illustrate examples for gas having a CO₂concentration of 1% or less.

FIG. 4 a graph representing CO₂ concentration ratios in the richsolution in test example 1. Herein, FIG. 4 represents a performanceimprovement ratio calculated based on standardized CO₂ concentration (1)without the discharge at each stage. In FIG. 4 , the horizontal axisrepresents the separation flow ratio of the flow rate (f1) of thesemi-rich solution 13B (circulation solution 13B) circulated through thefirst absorbent circulation line L₁₁ to the flow rate (f2) of thedischarged solution 13B-1 supplied to the absorbent discharge line L₂₁,in the range of 1:1 to 60:1 (the same applies to FIGS. 5 and 6 ). Thesame applies to the separation flow ratio of the flow rate (f1) of thecirculation solution 13C-1 circulated through the second absorbentcirculation line L₁₂ to the flow rate (f2) of the rich solution 13Csupplied to the rich solution supply line L₁, in the range of 1:1 to60:1.

FIG. 5 is a graph representing CO₂ recovery amount ratios in testexample 1. Herein, FIG. 5 represents a performance improvement ratiocalculated based on standardized CO₂ recovery amount (1) without thedischarge at each stage. FIG. 6 is a graph representing reductionpercentages (%) of reboiler heat duty in the absorbent regenerator intest example 1. FIG. 6 represents the reduction percentages (%)calculated based on standardized reboiler heat duty reductionpercentages (0) without the discharge.

As illustrated in FIGS. 4 to 6 , in test example 1, the separation flowratio in the range of 1:1 to 60:1 allows for improvement in theseparation effects on the CO₂ concentration ratio in the rich solution,the CO₂ recovery amount ratio, and the reboiler heat duty reductionpercentage (%). In particular, the separation flow ratio in the range of1:1 to 40:1 allows for improvement in the increase ratio of theseparation effects on the CO₂ concentration ratio in the rich solution,the CO₂ recovery amount ratio, and the reboiler heat duty reductionpercentage (%).

Here, the CO₂ absorption rate of the absorbent is represented as theproduct of the volumetric mass transfer coefficient (the product of themass transfer coefficient and the gas-liquid contact area) and thedriving force (the difference between the CO₂ partial pressure and theCO₂ equilibrium pressure). In the case of returning the absorbent of thesemi-rich solution 13B (circulation solution 13B) circulating from therich solution side at the bottom side of the CO₂ absorber 14 to the topof the CO₂ absorber 14, as disclosed in the prior art (Patent Literature1), the concentration of acid gas (CO₂ ) may be increased at the outletside due to decrease in the driving force when the CO₂ content is highin the gas (such as boiler flue gas with a CO₂ concentration of morethan 10% and 15% or less).

In contrast, when CO₂ is removed from the gas with a low CO₂concentration of 1% such as atmospheric air as in this test example,even if the absorbent is circulated from the rich solution side on thebottom side of the CO₂ absorber to the lean solution side on the upperside of the CO₂ absorber including the top thereof, it is possible toimprove the volumetric mass transfer coefficient to such a great extentas to outweigh the decrease of the driving force, and thereby improvethe CO₂ absorption rate. Thus, the improvement in CO₂ absorption rate inthe present invention is particularly noticeable under conditions withthe relatively low CO₂ concentration in the target gas.

Therefore, examples of the gas with low CO₂ concentration applicable tothe present invention include a gas at nearly atmospheric pressurecontaining a subtle amount (with a CO₂ concentration of 1% or less),preferably a gas at nearly atmospheric pressure with a CO₂ concentrationof 10% or less, more preferably a gas at nearly atmospheric pressurewith a CO₂ concentration of 5% or less. Examples of the gas with a CO₂concentration of 10% or less include flue gases from boilers, gasturbines, combustion furnaces, heating furnaces, incinerators, internalcombustion engines, and the like, for example, as well as atmosphericair and air in closed and substantially closed spaces.

In the present invention, in the case of targeting air, or air in theclosed or substantially closed space, the lean absorbent temperature isdesired to be same as or as close as possible to the target gastemperature.

When the introduced gas 11 in the CO₂ absorber 14 is atmospheric gas orair with unsaturated water vapor, the water content of the absorbent isevaporated and then exhausted as moisture saturated gas, therebyrequiring water to be supplied to the absorbent. The higher the leansolution temperature, the higher amount of water supplied to theabsorbent, due to increase in the temperature of the outlet gas 11C ofthe CO₂ absorber 14 and increased amount of exhausted water contentresulting from increase of saturated water content. For this reason, asillustrated in FIG. 1 , the lean solution supply line L₂ is providedwith a supply water introduction line L₉ to introduce supplied water 70to dilute the absorbent in the present embodiment, preventing the amineconcentration from increased in the absorbent.

The amount of absorbed CO₂ is small when the introduced gas 11A in theCO₂ absorber 14 is atmospheric gas or air with unsaturated water vapor.Thus, the amount of heat absorption resulting from the evaporativelatent heat of water content is greater than the amount of heatgenerated by the reaction when CO₂ is absorbed from the flue gasexhausted from the boiler as in the conventional system. Therefore, evenwhen the lean solution 13A with the same temperature as the introducedgas 11A is used, for example, the outlet gas temperature (T₂) of theoutlet gas 11C is lower by approximately 3° C. than that (T₁) of theintroduced gas 11A under the condition that the gas temperature (T₁) ofthe introduced gas 11A in test example 1 according to first embodimentis 25° C.

Thus, under the condition that CO₂ is removed from a gas containing asubtle amount of CO₂ (a CO₂ concentration of 1% or less), thetemperature of the circulation solution in the absorption section islower than that of the lean solution 13A introduced into the CO₂absorber 14, without a cooler installed in the first absorbentcirculation line L₁₁ and the second absorbent circulation line L₁₂. As aresult, the reaction heat of the amine solution is not generated whenCO₂ is absorbed inside the absorber as in the prior arts, thereby notrequiring the cooler in the first absorbent circulation line L₁₁ and thesecond absorbent circulation line L₁₂ in which the semi-rich solution13B and the rich solution 13C are circulated.

FIGS. 7 to 9 illustrate performances with comparison to the prior art(Patent Literature 2: International Publication No. 09/104744) in whichonly the second absorbent circulation line L₁₂ is used to circulate theabsorbent, without the first absorbent circulation line L₁₁ installed asin the present invention, (with circulation at the lower stage withoutcirculation at the upper stage) (Comparative Example 2).

FIG. 7 is a graph representing CO₂ concentration ratios in the richsolution in test example 2. Herein, FIG. 7 represents a performanceimprovement ratio calculated based on standardized CO₂ concentration (1)with circulation at the lower stage without circulation at the upperstage. In FIG. 7 , the horizontal axis represents the separation flowratio of the flow rate (f1) of the semi-rich solution 13B (circulationsolution 13B) circulated through the first absorbent circulation lineL₁₁ to the flow rate (f2) of the discharged solution 13B-1 supplied tothe absorbent discharge line L₂₁, in the range of 1:1 to 60:1 (the sameapplies to FIGS. 9 and 10 ). The same applies to the separation flowratio of the flow rate (f1) of the circulation solution 13C-1 circulatedthrough the second absorbent circulation line L₁₂ to the flow rate (f2)of the rich solution 13C supplied to the rich solution supply line L₁,in the range of 1:1 to 60:1.

FIG. 8 is a graph representing CO₂ recovery amount ratios in testexample 2. Herein, FIG. 8 represents the performance improvement ratiocalculated based on standardized CO₂ recovery amount (1) withcirculation at the lower stage without circulation at the upper stage.

FIG. 9 is a graph representing reduction percentages (%) of reboilerheat duty in the absorbent regenerator in test example 2. FIG. 9represents the reduction percentages (%) calculated based onstandardized reboiler heat duty reduction percentages (0) withcirculation at the lower stage without circulation at the upper stage.

As illustrated in FIGS. 7 to 9 , in test example 2, the separation ratioof the flow rates in the range of 1:1 to 60:1 allows for improvement inthe separation effects on the CO₂ concentration ratio in the richsolution, the CO₂ recovery amount ratio, and the reboiler heat dutyreduction percentage (%). In particular, the separation ratio of theflow rates in the range of 1:1 to 30:1 allows for improvement in theincrease ratio of the separation effects on the CO₂ concentration ratioin the rich solution, the CO₂ recovery amount ratio, and the reboilerheat duty reduction percentage (%).

Although explanations are given for the CO₂ recovery unit 10A accordingto the present embodiment in which two absorption stages (the firstabsorption stage 141A and the second absorption stage 141B) are providedin the CO₂ absorber 14, the present invention is not limited to this.Two or more absorption stages are provided (another absorption stage isprovided between the first absorption stage 141A and the secondabsorption stage 141B) in the CO₂ absorber 14 such that the circulationand discharge are performed at the predetermined separation ratio of theflow rates to circulate the absorbent.

Second Embodiment

FIG. 2 is a schematic view representing configuration of a CO₂ recoveryunit according to a second embodiment. Descriptions are given for newconfiguration in the second embodiment added to the CO₂ recovery unit10A according to the first embodiment illustrated in FIG. 1 . The samecomponents as in the configuration according to the first embodiment arenot described.

A CO₂ recovery unit 10B according to the second embodiment is providedwith a wash section 142 that is added inside the top part 14 a of thefirst absorption stage 141A of the CO₂ absorber 14 on a downstream sideof the gas flow in the CO₂ recovery unit 10A according to the firstembodiment.

The wash section 142 is provided at its bottom with a liquid reservoir144A that stores therein washing water W₂ to wash the decarbonatedoutlet gas 11C. A circulation line L₃ is provided between the liquidreservoir 144A and the top of the wash section 142 to supply wash waterW₂ containing the CO₂ absorbent collected in the liquid reservoir 144Afor circulation from the top part 14 a side of the wash section 142. Thecirculation line L₃ is equipped with a heat exchanger 21 that cools thewash water W₂ and a circulation pump 22 that circulates the wash waterW₂ containing the CO₂ absorbent collected in the liquid reservoir 144Athrough the heat exchanger 21 in the circulation line L₃.

In the present embodiment, CO₂ is recovered in the gas with low CO₂concentration, thereby the CO₂ absorption does not involve theexothermic reaction of the amine-based absorbent, as described above.However, the water wash section 142 illustrated in FIG. 2 is preferablyprovided to minimize loss of the amine-based absorbent in the outlet gas11C discharged outwardly, from the perspective of achieving near zeroemission.

Third Embodiment

FIG. 3 is a schematic view representing configuration of a CO₂ recoveryunit according to a third embodiment. Descriptions are given for newconfiguration in the third embodiment added to the CO₂ recovery unit 10Baccording to the second embodiment illustrated in FIG. 2 . The samecomponents as in the configuration according to the first embodiment arenot described.

A CO₂ recovery unit 10C according to the third embodiment is providedwith a gas quencher 12, which is added on an upstream side of the CO₂absorber 14 in the CO₂ recovery unit 10B according to the secondembodiment, to cool the introduced gas 11A down to a predeterminedtemperature. The gas quencher 12 has a quencher section 121 to cool theintroduced gas 11A. A water circulation line L₇ is provided to circulatethe cooling water W₁ between the bottom 12 b side of the gas quencher 12and the top 12 a side of the quencher section 121. The water circulationline L₇ is equipped with a heat exchanger 122 that cools the coolingwater W₁ and a circulation pump 123 that circulates the cooling water W₁in the water circulation line L₇.

In the quencher section 121, the introduced gas 11A is brought intocountercurrent contact with the cooling water W₁ so as to be cooled downto a predetermined temperature. The heat exchanger 122 cools the coolingwater W₁ heated by heat exchange with the introduced gas 11A. Thecirculation pump 123 supplies the cooling water W₁ flowing down to thebottom 12 b of the gas quencher 12 via the heat exchanger 122 to the top12 a of the quencher section 121. After cooled, the introduced gas 11Ais introduced into the CO₂ absorber 14 from the vicinity of the bottomof the CO₂ absorber 14 via the introduction line L₈.

In the third embodiment, the cooling part 55, which is installed in thelean solution supply line L₂ in the preceding configuration, isinstalled in the first absorbent circulation line L₁₁. This allows forcooling of the mixed solution (13A/13B) of the semi-rich solution 13B(circulation solution 13B) circulating from the first absorbentcirculation line L₁₁ and the lean solution 13A flowed from the leansolution supply line L₂.

The present embodiment differs from the first embodiment in that it ispreferable to aim for gases (e.g., turbine flue gas and various fluegases) containing 3% or more and 10% or less of CO₂ as the introducedgas 11A. For the gases containing 3% or more of CO₂, the liquidtemperature of the lean solution 13A is expected to be a lean absorbenttemperature at a temperature level of the water vapor-saturated gasflowed at the outlet of the gas quencher 12.

For example, under the condition that the gas temperature of theintroduced gas 11A in the present embodiment is 40° C., when theabsorbent is not circulated at each stage as in the prior art, the gastemperature of the outlet gas 11C discharged from the top 14 a isapproximately 10° C. higher than the gas temperature of the introducedgas. Therefore, under such a condition, it is more advantageous toinstall the cooling part 55 in the first absorbent circulation line L₁₁to prevent the temperature of the absorbent from rising, for increasingthe CO₂ concentration in the absorbent.

Hereinafter, preferred test examples exhibiting effects of theseparation flow ratio according to the present embodiment are described.The present invention is not limited to these examples. FIGS. 10 to 15of the test examples illustrate examples aimed for gases with a CO₂concentration of 3% or more.

FIG. 10 a graph representing CO₂ concentration ratios in the richsolution in test example 3. Herein, FIG. 10 represents a performanceimprovement ratio calculated based on standardized CO₂ concentration (1)without the discharge at each stage. In FIG. 10 , the horizontal axisrepresents the separation flow ratio of the flow rate (f1) of thesemi-rich solution 13B (circulation solution 13B) circulated through thefirst absorbent circulation line L₁₁ to the flow rate (f2) of thedischarged solution 13B-1 supplied to the absorbent discharge line L₂₁,in the range of 1:1 to 60:1 (the same applies to FIGS. 11 and 12 ). Thesame applies to the separation flow ratio of the flow rate (f1) of thecirculation solution 13C-1 circulated through the second absorbentcirculation line L₁₂ to the flow rate (f2) of the rich solution 13Csupplied to the rich solution supply line L₁, in the range of 1:1 to60:1.

FIG. 11 is a graph representing CO₂ recovery amount ratios in testexample 3. Herein, FIG. 11 represents a performance improvement ratiocalculated based on standardized CO₂ recovery amount (1) without thedischarge at each stage. FIG. 12 is a graph representing reductionpercentages (%) of reboiler heat duty in the absorbent regenerator intest example 3. FIG. 13 represents the reduction percentages (%)calculated based on standardized reboiler heat duty reductionpercentages (0) without the discharge.

As illustrated in FIGS. 10 to 12 , in test example 3, the separationflow ratio in the range of 1:1 to 60:1 allows for increase of theseparation effects on the CO₂ concentration ratio in the rich solution,the CO₂ recovery amount ratio, and the reboiler heat duty reductionpercentage (%). In particular, the separation flow ratio in the range of1:1 to 20:1 allows for improvement in the increase ratio of theseparation effects on the CO₂ concentration ratio in the rich solution,the CO₂ recovery amount ratio, and the reboiler heat duty reductionpercentage (%).

FIGS. 13 to 15 illustrate performances with comparison to the prior art(Patent Literature 2: International Publication No. 09/104744) in whichonly the second absorbent circulation line L₁₂ is used to circulate theabsorbent, without the first absorbent circulation line L₁₁ installed asin the present invention, (with circulation at the lower stage withoutcirculation at the upper stage) (Comparative Example 2).

FIG. 13 is a graph representing CO₂ concentration ratios in the richsolution in test example 4. Herein, FIG. 13 represents a performanceimprovement ratio calculated based on standardized CO₂ concentration (1)with circulation at the lower stage without circulation at the upperstage. In FIG. 7 , the horizontal axis represents the separation flowratio of the flow rate (f1) of the semi-rich solution 13B (circulationsolution 13B) circulated through the first absorbent circulation lineL₁₁ to the flow rate (f2) of the discharged solution 13B-1 supplied tothe absorbent discharge line L₂₁, in the range of 1:1 to 60:1 (the sameapplies to FIGS. 14 and 15 ). The same applies to the separation flowratio of the flow rate (f1) of the circulation solution 13C-1 circulatedthrough the second absorbent circulation line L₁₂ to the flow rate (f2)of the rich solution 13C supplied to the rich solution supply line L₁,in the range of 1:1 to 60:1.

FIG. 14 is a graph representing CO₂ recovery amount ratios in testexample 4. Herein, FIG. 14 represents the performance improvement ratiocalculated based on standardized CO₂ recovery amount (1) withcirculation at the lower stage without circulation at the upper stage.FIG. 15 is a graph representing reduction percentages (%) of reboilerheat duty in the absorbent regenerator in test example 4. FIG. 15represents the reduction percentages (%) calculated based onstandardized the reboiler heat duty reduction percentages (0) withcirculation at the lower stage without circulation at the upper stage.

As illustrated in FIGS. 13 to 15 , in test example 4, the separationratio of the flow rates in the range of 1:1 to 60:1 allows forimprovement of the separation effects on the CO₂ concentration ratio inthe rich solution, the CO₂ recovery amount ratio, and the reboiler heatduty reduction percentage (%). In particular, the separation ratio ofthe flow rates in the range of 1:1 to 40:1 allows for improvement in theincrease ratio of the separation effects on the CO₂ concentration ratioin the rich solution, the CO₂ recovery amount ratio, and the reboilerheat duty reduction percentage (%).

The CO₂ recovery unit and the CO₂ recovery method described in theembodiments are understood, for example, as follows.

The CO₂ recovery unit 10A, 10B, 10C according to a first aspectincludes: the CO₂ absorber 14 configured to bring the introduced gas 11Ahaving a low CO₂ concentration into countercurrent contact with the CO₂absorbent and remove CO₂; the absorbent regenerator 15 configured forheat exchange between the rich solution 13C containing absorbed CO₂ andthe vapor flowed from the reboiler 31 to regenerate the CO₂ absorbent;the rich solution supply line L₁ configured to discharge the richsolution 13C containing absorbed CO₂ from the bottom 14 b of the CO₂absorber 14 and supply the rich solution to the top side of theabsorbent regenerator 15; and the lean solution supply line L₂configured to discharge the lean solution 13A, which is obtained afterreleasing CO₂, from the bottom 15 b of the absorbent regenerator 15, andsupply it to the top side of the CO₂ absorber 14, in which the CO₂absorber 14 has at least two or more CO₂ absorption stages 141A, 141B.The CO₂ recovery unit 10A further includes: the absorbent circulationlines L₁₁, L₁₂ configured to discharge the CO₂ absorbent from the lowersides of the CO₂ absorption stages 141A, 141B and supply the CO₂absorbent as the circulating semi-rich solution 13B (circulationsolution 13B) and the circulating rich solution 13C (circulationsolution 13C-1) to the upper sides of the corresponding CO₂ absorptionstages 141A, 141B that are used as the discharge stages; and theabsorbent discharge lines L₂₁, L₁ configured to discharge parts of thesemi-rich solution 13B (circulation solution 13B) circulation solution13C-1 from the absorbent circulation lines L₁₁, L₁₂ and supply thesolution as the discharged solutions 13B-1, 13C to adjacent stageimmediately below the discharge stages.

This configuration enables it to discharge the CO₂ absorbent containingthe absorbed CO₂ from the lower portion of each of the CO₂ absorptionstages, supply it to the upper portion of the corresponding stage atwhich the CO₂ absorbent is discharged, and reuse the solution as the CO₂absorbent for the purpose of recovering CO₂ in the gas, increasing theCO₂ concentration in the CO₂ absorbent, and reducing the reboiler heatduty needed for recovering CO₂ from a variety of gases with low CO₂contents.

The CO₂ recovery unit 10A, 10B, 10C according to a second aspect isaimed for the introduced gas 11A with a CO₂ concentration of 10% byvolume or less.

This configuration enables it to reduce the reboiler heat duty neededfor recovering CO₂ from the introduced gas 11A with a CO₂ concentrationof 10% by volume or less.

In the CO₂ recovery unit 10A, 10B, 10C according to a third aspect, theflow ratio is set at 1:1 to 60:1 for the separation ratio of the flowrates of the circulation solution 13B, 13C-1 to the discharged solution13B-1, 13C.

With the separation ratio of the flow rates of the circulation solution13B, 13C-1 to the discharged solution 13B-1, 13C ranging from 1:1 to60:1 in this configuration, it is possible to reduce the reboiler heatduty needed for recovering CO₂ from a variety of gases with low CO₂contents.

The CO₂ recovery unit 10B, 10C according to a fourth aspect is providedwith the wash section 142 at the downstream side of gas flow in the CO₂absorption section at the highest stage.

This configuration having the wash section 142 enables it to wash theoutlet gas 11C with water, and thereby minimize the loss of theamine-based absorbent in the outlet gas 11C released outwardly.

The CO₂ recovery unit 10C according to a fifth aspect has the coolingpart 55 in the first absorbent circulation line L₁₁.

With the cooling part 55 in the first absorbent circulation line L₁₁,this configuration enables it to prevent the temperature of theabsorbent from increasing, achieving advantageous effects on theincrease in the CO₂ concentration in the absorbent.

A CO₂ recovery method according to a sixth aspect includes: a CO₂absorption step of bringing the introduced gas 11A having low CO₂concentration into countercurrent contact with the CO₂ absorbent toremove CO₂; an absorbent regeneration step of performing the heatexchange between the rich solution 13C containing the absorbed CO₂ andthe steam flowed from the reboiler 31 to regenerate the CO₂ absorbent;and a step of discharging the rich solution 13C containing CO₂ that isabsorbed at the CO₂ absorption step, supplying it to the absorbentregeneration step, discharging the lean solution 13A that is obtained byreleasing CO₂ at the absorbent regeneration step, and supplying it tothe CO₂ absorption step to recover CO₂ in the gas. The CO₂ absorptionstep includes an absorbent circulation step of, in the CO₂ absorptionsection with at least two or more stages 141A, 141B, discharging the CO₂absorbent from the lower sides of the CO₂ absorption stages 141A, 141Band then supplying it as the circulation solution 13B, 13C-1 to theupper sides of the corresponding CO₂ absorption stages 141A, 141B thatare used for the discharge; and an absorbent discharge step ofdischarging a part of the circulation solution 13B from the absorbentcirculation step and supplying it as the discharged solution 13B-1, 13Cto the stage immediately below the stage used for the discharge.

This configuration enables it to discharge the CO₂ absorbent containingthe absorbed CO₂ from the lower portion of each of the CO₂ absorptionstages, supply it to the upper portion of the corresponding stage atwhich the CO₂ absorbent is discharged, and reuse the solution as the CO₂absorbent for the purpose of recovering CO₂ in the gas, increasing theCO₂ concentration in the CO₂ absorbent, and reducing the reboiler heatduty needed for recovering CO₂ from a variety of gases with low CO₂contents.

REFERENCE SIGNS LIST

10A to 10C CO₂ recovery unit

11A, 11B Introduced gas

11C Outlet gas

13A Lean solution

13B Semi-rich solution

13B-1 Discharged solution

13C Rich solution

13C-1 Circulation solution

14 CO₂ absorber

15 Absorbent regenerator

31 Reboiler

55 Cooling part

141A First CO₂ absorption stage (first absorption stage)

141B Second CO₂ absorption stage (second absorption stage)

144 Liquid reservoir

L₁ Rich solution supply line

L₂ Lean solution supply line

L₁₁ First absorbent circulation line

L₁₂ Second absorbent circulation line

L₂₁ Absorbent discharge line

1. A CO₂ recovery unit comprising: a CO₂ absorber that brings a gashaving a low CO₂ concentration into countercurrent contact with a CO₂absorbent to remove CO₂ from the gas; an absorbent regenerator thatperforms a heat exchange between a rich solution containing absorbed CO₂and water vapor flowed from a reboiler to regenerate the CO₂ absorbent;a rich solution supply line that discharges a rich solution in which CO₂has been absorbed by the CO₂ absorbent in the CO₂ absorber, from abottom part of the CO₂ absorber and supplies the rich solution to anupper side of the absorbent regenerator; and a lean solution supply linethat discharges a lean solution in which CO₂ has been released from therich solution in the absorbent regenerator, from a bottom part of theabsorbent regenerator and supplies the lean solution as the CO₂absorbent to an upper side of the CO₂ absorber, wherein the CO₂ absorberincludes at least two or more CO₂ absorption sections including a firstCO₂ absorption section and a second CO₂ absorption section located belowthe first CO₂ absorption section, and the CO₂ recovery unit furthercomprises: a first absorbent circulation line that discharges the CO₂absorbent from the first CO₂ absorption section and supplies the CO₂absorbent as a first circulation solution to an upper side of the firstCO₂ absorption section; a second absorbent circulation line thatdischarges the CO₂ absorbent from the second CO₂ absorption section andsupplies the CO₂ absorbent as a second circulation solution to an upperside of the second CO₂ absorption section; and an absorbent dischargeline that discharges a part of the first circulation solution from thefirst absorbent circulation line and supply the part of the firstcirculation solution as a discharged solution to the second absorbentcirculation line.
 2. The CO₂ recovery unit according to claim 1, whereinthe gas has a CO₂ concentration of 10% by volume or less.
 3. The CO₂recovery unit according to claim 1, wherein a separation ratio of flowrates of the first circulation solution to the discharged solution isset to 1:1 to 60:1.
 4. The CO₂ recovery unit according to claim 1,further comprising a water wash section on a downstream side of a gasflow in the first CO₂ absorption section.
 5. The CO₂ recovery unitaccording to claim 1, wherein the first absorbent circulation lineincludes a cooling part.
 6. A CO₂ recovery method comprising: a CO₂absorption step of bringing a gas having a low CO₂ concentration intocountercurrent contact with a CO₂ absorbent to remove CO₂ from the gas;an absorbent regeneration step of performing a heat exchange betweenwater vapor and a rich solution containing absorbed CO₂ to regeneratethe CO₂ absorbent; and a step of discharging a rich solution in whichCO₂ has been absorbed at the CO₂ absorption step, supplying the richsolution to the absorbent regeneration step, discharging a lean solutionin which CO₂ has been released from the rich solution at the absorbentregeneration step, and supplying the lean solution as the CO₂ absorbentto the CO₂ absorption step to recover CO₂ in the gas, wherein the CO₂absorption step includes at least two or more CO₂ absorption stepsincluding a first CO₂ absorption step and a second CO₂ absorption stepfollowing the first CO₂ absorption step, the CO₂ recovery method furthercomprises: a first absorbent circulation step of discharging the CO₂absorbent obtained at the first CO₂ absorption step and supplying theCO₂ absorbent as a first circulation solution to the first CO₂absorption step; and a second absorbent circulation step of dischargingthe CO₂ absorbent obtained at the second CO₂ absorption step andsupplying the CO₂ absorbent as a second circulation solution to thesecond CO₂ absorption step; and an absorbent discharge step ofdischarging a part of the first circulation solution obtained at thefirst absorbent circulation step and supplying the part of the firstcirculation solution as a discharged solution to the second absorbentcirculation step.